1. Field of the Invention
The present invention is directed to an electrical multilayer component, particularly a capacitor, a PTC resistor or a varistor.
The multilayer component of the present invention includes a sintered, monolithic ceramic body having dielectric or semiconductive properties. Cavities are provided which alternate from layer to layer and are open alternately toward end faces which are opposed to each and are open to the adjacent lateral surfaces. Ceramic particles are distributed in the cavities, and serve to provide supporting elements in the cavities between the adjoining ceramic layers. The cavities are filled with a metal filler which is composed of a metal or alloy whose melting temperature is considerably lower than the temperature required for sintering of the ceramic body.
Contacts are applied at the end faces and may extend partially around on to the lateral faces, the contact members connecting the metal fillers to one another in electrically conductive fashion and being solderable, per se, or being provided with a coating which is solderable.
The invention also is directed to a method for the manufacture of such electrical multilayer components.
2. DESCRIPTION OF THE PRIOR ART
Electrical multilayer capacitors having a sintered, monolithic ceramic body of dielectric material of the type with which the present invention is concerned have been disclosed in the patent literature in many embodiments. Of this extensive patent literature, U.S. Pat. Nos. 3,679,950; 4,071,880; and 4,658,328 which corresponds to German published application No. 36 12 084 appear to be the most pertinent to the present invention.
U.S. Pat. No. 3,679,950 discloses a ceramic multi-layer capacitor which provides a multilayer capacitor except that the cavities formed therein which alternate from layer to layer are open toward end faces which lie opposite to one another but are not open toward the adjacent lateral surfaces. This U.S. patent also discloses a method for the manufacture of such an electrical multilayer capacitor but employs cavities which are open only to one side. Since the plurality of layers made up in this way forms a stack which is practically closed on all sides, it is necessary to separate the stack into discrete members to permit escape and/or decomposition of the agents used for forming the cavities in a subsequent sintering process for which an appropriate heating, sintering and cooling program is required.
In addition to other possibilities, a metal having a low melting point or an alloy having a low melting point, for example, lead or an alloy composed of 50% Bi, 25% Pb, 12.5% Sn and 12.5% Cd have been suggested for the manufacture of the capacitor coatings inside the cavities of the monolithic ceramic body. All of these alloys have melting temperatures which are considerably lower than the sintering temperature of about 1325.degree. C. required for sintering the monolithic ceramic body. These alloys do not wet the surface of the ceramic body, particularly in the cavities, or only wet it poorly.
With the multilayer capacitor disclosed in U.S. Pat. No. 3,679,950 the contact layers which are usually composed of silver are always applied to the end faces which lie opposite one another after the molten metal having a low melting point has been pressed in and after cooling of the saturated ceramic body.
U.S. Pat. No. 4,071,880 represents an improvement on U.S. Pat. No. 3,679,950 and is directed to an electrical multilayer capacitor comprising a sintered, monolithic body of the type described in the previously mentioned patent. U.S. Pat. No. 4,071,880 also discloses the possibility in detail of applying contact layers to the end faces of the monolithic ceramic body before its saturation, for which purpose the contact layers must be made porous.
The multilayer capacitor disclosed in U.S. Pat. No. 4,071,880 is composed of a plurality of ceramic layers and inside electrodes which lie on top of one another in alternating form and is also composed of outer double layer electrodes constituting contact layers which are connected to the inside electrodes in a predetermined fashion. For example, a comb-like electrode structure may be provided.
In the manufacture of such a monolithic multilayer capacitor, an untreated or unsintered ceramic material in the form of laminae is first produced by means of a stripping method using a stripper or scraper blade. These ceramic laminae have a thickness of about 0.05 to 0.1 mm. A carbonaceous paste is then applied or printed on to the surfaces of the ceramic laminae, the paste being composed of a mixture of a carbon powder and a ceramic powder. This paste is applied in limited regions such that the cavities in the finished ceramic body are alternately open only toward opposite end faces. A plurality of such printed ceramic laminae are arranged on top of one another in alternate form and are joined to one another for forming an integrated structure by applying pressure. Discrete bodies are produced from this integrated structure by cutting along cutting lines that extend perpendicularly relative to one another, the discrete bodies being sintered at a temperature above 1000.degree. C. in order to sinter the ceramic laminae and in order to eliminate the carbon powder within the carbon paste which has been printed on. As a result, porous intermediate layers comprising ceramic powder are produced in regions in which the inside electrodes are to be formed. Following this, the porous outer electrodes or contact layers are applied to the sintered discrete bodies.
These structures can be produced, as well as structures of the present invention, utilizing methods that are disclosed in U.S. Pat. Nos. 3,683,849; 4,526,129; 4,561,954; British Patent No. 2 106 714; German Patent No. 27,22,140, or German Published Application No. 36 38 286.
For example, the outer electrodes can be obtained by firing a paste that is principally composed of nickel and is mixed with a glazing compound. The ceramic body formed in this way is then introduced into a pressure vessel and is dipped into molten lead which serves as the conducting material for the capacitor coatings, the temperature of the melt being about 330.degree. to 360.degree. C. Subsequently, the pressure is elevated to about 10 bar so that molten lead penetrates into the cavities of the ceramic body under pressure, passing exclusively through the porous, outer electrodes. The ceramic body is then removed from the molten lead, cooled, and again exposed to normal pressure so that the inner electrodes of lead are formed. Following this, additional layers that are solderable are applied to the outer electrodes.
In order to manufacture the described multilayer capacitor, the outer electrodes applied to the monolithic ceramic body must be porous and must be fashioned or manufactured such that entry of the molten lead is initially possible for the inside electrodes but, on the other hand, they prevent the lead from flowing out of the cavities of the ceramic body when it is removed from the molten lead. As explained in U.S. Pat. No. 4,071,880, in column 10, lines 51 through 58, the metal melt should not easily wet the ceramic body. In other words, the porous outer electrodes form penetration barriers in certain conditions. The porous, outer electrodes are principally composed of nickel that does not react with lead. The adhesion to the end faces of the ceramic body depends on the amount of glazing compound within the paste, the adhesion being improved with an increasing amount of glazing compound. In this case, however, the number of pores in the outer electrode is diminished so that the penetration of the lead is rendered more difficult, whereas glass components block the intermediate layers and deteriorate the delivery of lead. The electrostatic capacitance can therefore not be set in a desired way, even when the ceramic module is exposed to a relatively high pressure within the melt.
In order to overcome the problems described, U.S. Pat. No. 4,658,328 which corresponds to German Published Application No. 36 12 084 has proposed that the cavities in the interior of the ceramic body be designed such that they are open both to end faces which lie opposite one another as well as, to a slight degree, toward the adjoining lateral surfaces so that the molten metal in the injection process can penetrate not only through the porous contact layers but also can penetrate through the lateral surfaces that are open to a slight degree and through parts of the cavities that are free of contact layers, and can nevertheless not flow out from the cavities again.
U.S. Pat. No. 4,658,238 also discloses a method which comprises the above recited method steps for the manufacture of the capacitor, but with the requirement that the contact layers applied to the end faces continue to be porous.
German Published Application Nos. 36 27 936 which corresponds to U.S. Ser. No. 086,520, filed Aug. 18, 1987, and 36 27 928 disclose multilayer capacitors comprising a sintered, monolithic ceramic body and disclose methods for the manufacture thereof which likewise comprise the features of the previously described multilayer capacitors and the methods for their manufacture. In contrast with the metal or metal alloys heretofore proposed for these purposes, these applications propose the use of a metal which wets the surface parts inside the cavities of the ceramic body. A series of proposals that are also valid for the metal or metal alloys suitable for the present invention are made regarding these metals or alloys.
According to these applications, pressing the metals into the ceramic bodies continues to occur through the openings of the cavities that are directed toward only one side and can either still be free of a contact or be covered by a porous contact layer. In the former case, the contact layers are subsequently applied.
The use of a metal which wets the surface of the ceramic body well has the advantage that the metal no longer flows out of the cavities of the ceramic body when it is removed from the metal melt and a good bonding of the capacitor coatings to the contacts is guaranteed.
When the easily melting metal alloys are pressed through porous contact layers into the cavities of the ceramic body, the cavities being open only toward one side, the problems that have already been set forth appear, namely, the risk of blocking the pores in the porous layer and thus the risk of an inadequate filling of the cavities. These problems are incapable of being eliminated even when employing better wetting metals or metal alloys. The problems again arise when applying the contacts following the impression of the metal melt and cooling of the filled ceramic bodies, these problems being concerned with the bonding of the capacitor coatings in the inside of the ceramic body to the contacts at the end faces being inadequate.
Ceramic PTC resistors in the format of layers are disclosed, for example, by British Patent No. 932 558 and ceramic multilayer varistors are disclosed, for example, in U.S. Pat. No. 4,675,644 which corresponds to European Patent Application No. 0 189 087.